I do not believe that the problem is with the PB machine. I have numerous customers with SLT ports going out on PRI circuits just fine. However, I do have a few sites where modems (credit card, postage and fax machine, dial back-up) do not connect and fail during handshake or shortly thereafter. Faxes will transmit partially then drop. And yes, we have played with modem settings, baud rates, switching PRI cards, etc... I would recommend talking to the carrier/tester. Other than that, go to analog.
